A typical USB device supports several USB connection speeds to achieve downward compatibility for the USB host platform, such as connection specifications that support Super speed plus (USB 3.1 Gen2), Super speed (USB 3.1 Gen1), and High Speed (USB 2.0). When a typical USB device is connected to the USB host platform, the USB device preferentially starts with the highest speed of USB connection speed to communicate with the USB host. The higher the USB connection speed is, the faster the data transfer is, and the greater the power consumption is required by the USB device.
Generally, the new USB host device (e.g., a notebook, a tablet, a mobile phone, and the like) can support a variety of USB connection speeds. When the USB device is connected, the final connection speed can be determined through the transmission protocol. When the USB host and the USB device both support the USB Super Speed Plus at the highest speed, the USB host and the USB device will communicate with each other at this speed and perform transmission.
However, many existing USB host platforms (e.g., tablets, mobile phones, and the like) have different power supply capabilities, and usually limit the power-carrying capability for the connected USB devices. Therefore, the currently connected USB device is not always allowed to be powered at high connection speed. When the USB device requires power greater than the power capability supported by the USB host platform, the USB device is likely to be powered-off. After the power is turned on again, the USB device will be powered off again since the connection speed is still at the highest speed, such that a dead loop is formed and the USB device can never be used by the USB host.
Therefore, it has become one of the important issues in this field to overcome the above-mentioned issues by improving the mechanism for power supply.